Structure-activity relationship on cytotoxicity of rotenoid compounds

Abstract

6-Deoxyclitoriacetal (1) and stemonal (2) are rotenoid substances extracted from the dried root of Stemona collinse Craib. Only 1 has been known to have cytotoxic activity against various types of human carcinoma. Interestingly, although 1 is not a derivative of doxorubicin, it shares structural similarities with doxorubicin. In contrast, 2 has no cytotoxic acitivity against anticancer cell lines. These lead to the study of structure-activity relationship and the proposed hypothesis that a substance has a good chance to be a DNA-intercalating anticancer drug if it possesses three characteristics: (i) the molecule has a bent shape, (ii) a part of the molecule is planar and (iii) it has functional groups that have intermolecular interactions. The interaction between 1 and 2 to calf thymus DNA has been studied in detailed by means of UV - spectroscopy, denaturation temperature (Tm) and circular dichroism (CD). In addition, both 1 and 2 were studied for the binding with hexamer d(CGTACG)2. They are found to interact with base pairs of DNA as evidenced by (1) induced UV spectra; (2) increased denaturation temperature of the DNA helix; (3) induced circular dichroism spectra; (4) broadened and shifted 1H NMR signals. The nuclear magnetic resonance experiment shows that the planar aromatic ring of 1 and 2 intercalates between CG base pairs of DNA when it is bound to DNA. Compound 1 showed strong inhibitory topoisomerase II, giving 75.22 % inhibition that was better than etoposide (68.94 % inhibition). The derivatives of 1 have been synthesized and investigated for their cytotoxic activities and also topoisomerase II inhibition, to study the effect of the functional groups on the ability to stabilize the DNA complexes. Among the amino acid derivatives of 1 (A1 to A5), arginine derivative of 1 (A5) showed strong selective cytotoxic activity with KB with IC₅₀ of 1.45 g/ml. Among the pyrimidine base derivatives of 1 (B1 to B3), the uracil derivative (B3) showed strong cytotoxic activities against KB and NCI-H187 with IC₅₀ of 1.45 and 0.255 g/ml, respectively. Among the carboxylic aromatic ester derivatives of 1 (C to G), G showed strong cytotoxic activities against KB and NCI-H187 with IC₅₀ of 2.64 and 8.28 g/ml, respectively. Compound A1 to A5 showed moderate inhibitory topoisomerase II, giving 50.10, 35.60, 31.50, 30.20, 39.50 % inhibition, respectively. The results of topoisomerase II inhibition of A1 to A5 are consistent with the cytotoxicity activity. The results have confirmed the hypothesis.